Solid state cameras, just like the conventional film camera, are limited in their ability to take pictures which faithfully replicate the full range of colors and brightness in a scene. This is because natural scenes exhibit a wide dynamic range, i.e., some regions of a scene are very bright while others are very dark. As a result, conventional solid state cameras, and particularly consumer products such as digital cameras and video cameras, have a number of adjustable exposure parameters that control the sensitivity of a camera imager. The best pictures are usually taken after the camera's exposure parameters have been adjusted according to the amount of light in the scene. For instance, if the scene is relatively bright, then the exposure, e.g., the period of time the camera imager is allowed to “sense” the incident light, is accordingly reduced so as to better capture the brightness variations in the scene. In conventional solid state cameras, a separate light meter sensor and associated circuitry are used to quickly give an immediate luminance reading of the scene prior to adjusting the exposure parameters and then taking the picture. However, both the light metering circuitry and the camera imager must be calibrated, at the time of manufacturing the camera, to a reference light source. Otherwise, the technique may not yield the proper exposure parameters.
There is a limited conventional technique for determining the optimal exposure that does not use a separate light metering circuit. In that technique, the camera is equipped with a means for providing a histogram of the captured scene at a given exposure setting. The histogram shows a distribution of pixel values obtained by the imager at the selected exposure setting. A person can then manually change the exposure setting and then visually evaluate another histogram of the scene obtained using the new exposure setting. The exposure setting is repeatedly adjusted in this way until the optimal distribution of pixels has been obtained, and then the picture is taken using this optimal exposure setting. This technique suffers, however, when implemented in commercial solid state cameras, because it is too slow and is not automatic for the average consumer who likes the point and shoot convenience of automatic cameras.